1. Field of the Invention
The present invention relates generally to a pressure control circuit for a hydraulic circuit for controlling discharge pressure for a pair of hydraulic loads. More specifically, the invention relates to a pressure control valve which successfully prevent foreign matters which may cause blocking of a hydraulic circuit and/or hydraulic load from flowing into the hydraulic load. The present invention also relates to a technology for ventilating air introduced into the hydraulic circuit and minimizing effect of a residual air in the hydraulic circuit for damping vibration of valve spool.
2. Description of the Background Art
Pressure control valves have been employed in various hydraulic circuits for controlling hydraulic pressure distribution for loads. For example, one of typical construction of pressure control valves can be seen in "Uchida Lexloss direct type electromagnetic proportioning valve" shown in its catalog, May, 1986, published by K. K. Silver Design.
The conventionally proposed pressure control valve defines an induction port, a drain port, a first control port and a second control port, which ports are communicated with a valve bore defined in a valve body. A valve spool is thrustingly disposed within the valve bore for distributing working fluid introduced through the induction port to the first and second control ports. The valve spool is loaded by a pair of bias springs so as to be biased toward a neutral position. A pair of electrically or electromagnetically operable actuators are provided for driving the valve spool for causing an axial shift for controlling distribution of the working fluid to the first and second control ports.
Control current to be applied to the actuators is so controlled as to cause axial shifting of the valve spool for adjusting distribution of the pressure for the first and second control ports in an inversely proportional fashion. During pressure distribution control in such pressure control valve, control pressure to be supplied to respective loads by way of the first and second control ports varies according to non-linear characteristics in a second order curve. These variation characteristics of the control pressure are caused by a simultaneous variation of path areas at an introduction side and a drain side. Therefore, in case that a load is an actuator, activity of the actuator varies as a second order function of the valve spool stroke. This makes control of the actuator by supplying the control pressure difficult in that requiring a complex arithmetic operation becomes necessary. Particularly, at the valve spool position in the vicinity of a completely shutting position a variation of control pressure versus the valve spool stroke becomes substantial to cause an impulsive change in the actuator.
Furthermore, foreign matter may contained in the working fluid to circulate therewith through the hydraulic circuit and through small gaps defined between components of the pressure control valve assembly. Such foreign matter tends to enter into the small gaps between the components for providing resistance against movement of the components. For example, when the pressure control valve is provided a pilot piston for adjusting pilot pressure in the pressure control valve for determining the valve spool position so that the pilot pressure is cooperating with the mechanical spring force and the force exerted on the valve spool by means of an electromagnetic actuator are to be balanced, the foreign matter entering the small gap prevents the pilot valve from shifting smoothly to cause degradation of response characteristics.
Additionally, as can be appreciated, residual air in the hydraulic circuit often affects for performance of the pressure control valve. Particularly, when dither current is superimposed on a control current supplied to an elecromagnetic actuators so as to minimize resistance of movement of a valve spool, vibration of the valve spool caused by dither current tends to amplify noise when residual air is present in the hydraulic circuit or in the pressure control valve. Removal or ventilation of the residual air in the pressure control valve has been typically performed by removing a closure plug in a ventilation path. This process is cumbersome. In addition, a difficulty is encountered in removing the air when the air resides in the orientation remote from the ventilating opening of the ventilation path.